This invention relates to plastic containers, especially plastic containers for pressurized fluids, and more particularly, to an improved bottom structure for plastic bottles of the type suitable for containing effervescent or carbonated beverages.
Blow-molded plastic bottles for containing liquids at elevated pressures are known and have found increasing acceptance. Such containers are accepted particularly in the beverage industry for use as one-way disposable containers for use with effervescent or carbonated beverages, especially carbonated soft drinks. Plastic bottles of this type are subject to a number of structural and functional criteria which have presented many problems not previously solved. Solutions to the problems offered by the prior art have yielded bottles which are not entirely satisfactory.
Because many of the pieces of the equipment used in the handling and filling of such bottles are costly and were manufactured to work with glass bottles, attempts were made to conform the plastic bottles to the size and shape of prior art glass bottles employed for the same purpose. However, it has been found that a mere replication of the prior art glass bottles in plastic is not entirely satisfactory. The replication of the glass structure in plastic is not possible due to the resilient nature of the plastic materials and the distortion and creep which the plastic materials can exhibit at elevated pressure especially when such bottles are subjected to elevated temperatures. Further, the plastic bottle is limited to certain modification by the very nature of the blowing process and the available materials for use in forming such a bottle.
The overwhelming use for the bottles of this type are where the contained liquid will be carbonated. When used with carbonated beverages, the bottles may be subjected to internal pressures normally between 40 and 100 pounds per square inch and occasionally as high as 200 psi under severe conditions of elevated temperature, especially during transportation. In such a condition, the bottle is presented with an elevated pressure within the bottle when filled. This pressure, however, will be absent both prior to sealing and subsequent to the opening of the bottle. The potential for failure in the plastic bottle when pressurized is greatest at the bottom of the container. Various designs have been employed to effectively deal with this condition.
One of the initial plastic bottle designs had a bottom design consisting generally of a hemispherical bottom to which was added as a separate member a base cup which supports the bottle in an upright position. This design is shown for example, in U.S. Pat. No. 3,722,725. This design has been widely used and adopted in the industry. It provides a strong bottle because the hemispherical bottom is the geometric shape which most uniformly adapts to pressure. However, this basic design has several significant disadvantages.
Initially, the design requires the separate manufacture of the bottle and the base cup. It also requires the additional mechanical step of attaching the base cup to the bottle. In addition, the amount of material used in the bottle and in the base cup is beginning to cause concern among the ever more environmentally-conscious public. Compounding the environmental problem, in commercial embodiments, the bottle and base cup are generally made from dissimilar plastic materials. In such a case, the reclamation or recycling of the plastic used in the bottles is difficult if not impossible.
Due to the manufacturing and disposal problems inherent in the two-piece construction, the art turned to the manufacture of one-piece bottles. Such bottle designs have generally taken the form of bottles where the bottom design is a plurality of feet integrally formed in the base of the bottle upon which the bottle rests, for example U.S. Pat. No. 3,759,410. Other designs for one-piece bottles include a continuous peripheral seating ring upon which the bottle rests surrounding a generally concave central portion, e.g., U.S. Pat. No. 4,247,012.
In existing one-piece bottle bottom constructions three general problems have been identified in the art. Initially, such plastic bottles have not had enough bottom strength to withstand the impact of falling from a moderate height onto a hard surface when filled with a carbonated beverage. Further, because the bottles are often subjected to extreme temperatures, it has been found in some designs that the bottom of the bottle everts or otherwise distorts producing a bottle known in the industry as a "rocker" where the bottle wobbles in transportation or display. Finally, another problem is the stress cracking of such bottles, especially under extremes of temperature or pressure or when exposed to any stress cracking agent during filling, handling or transportation.
Moreover, as is known in the art, it is highly desirable that any bottle design be of a type which is aesthetically pleasing as the bottle's design is used as one feature in the marketing and sale of the contained liquid. One known bottom structure which is generally considered aesthetically pleasing is the so-called "champagne" bottom. Based upon the traditional design of glass champagne bottles, the champagne bottom has a central upwardly convex portion which extends up into the bottle interior from the continuous base which is a continuation from the bottle sidewall.
Polyethylene terephthalate (PET) is the preferred plastic used in the formation of bottles for carbonated beverages. PET is a desirable material to use in such bottles because, when properly processed it has the requisite clarity, strength, and resistance to pressure leakage necessary for such bottles. Specifically, when blow-molded, PET is essentially completely transparent. The PET material has sufficient gas barrier properties so that carbonated beverages can be stored for extended periods of time without losing any significant amount of the CO.sub.2 pressure given by carbonation. Commonly, bottles are blow molded from injection molded "preforms" of PET.
Blow molded bottles formed from injection molded preforms tend to have a particularly acute stress cracking problem in the area of the bottle bottom portion which includes and lies adjacent to the nib remaining on the preform from the sprue or "gate" through which the molten polymer is injected into the preform mold. This gate area is manifest in the blow-molded bottle by a clouded circlet at or very near the center of the bottle bottom. In the prior art bottles, this gate area contains far less biaxial orientation than is present in the bottle sidewall or in the remainder of the bottom. As a result of this deficiency, the gate area of a bottle blow molded from an injection molded preform is more likely to fail under stress, particularly under the extreme conditions experienced in the transportation and storage especially in geographical areas where the ambient temperature exceeds 100.degree. F., than other areas of the bottle sidewall and bottom. The beverage industry suffers substantial losses due to this stress-cracking problem.
Thus, the present invention provides a design for a blow-molded one-piece plastic beverage container having a bottom design overcoming the problems of the prior art. Specifically, the container of the present invention is strong enough to withstand a blow from a fall, will not evert under pressure, is resistant to stress cracking, and is aesthetically pleasing.